The technology for watermarking digital images is well developed. The intensity of the watermark determines both if the watermark will create visual artifacts and how easily the watermark will be to detect after an image has been printed, copied or otherwise transformed. When inserting a digital watermark into an image, it is desirable to utilize a high intensity watermark signal, that is, to change the intensity of the pixels in the image as much as possible so that it will be easy to detect and read the watermark. However, if the intensity of the watermark signal is too high, the watermark may create visual artifacts in the image, that is, the watermark signal may be visually evident.
Many watermarking technique are “perceptually adaptive”. Perceptually adaptive watermarking techniques take into account the characteristics of an image in an effort to make a watermark relatively easy to detect and to avoid causing visually apparent artifacts in the watermarked image.
A perceptually adaptive watermarking technique is for example shown in U.S. Pat. No. 5,748,763 entitled “Image Steganography System Featuring Perceptually Adaptive and Globally Scalable Signal Embedding”by Geoffrey B. Rhoads. Certain commercially available image editing programs (such as the image edition program marketed by Adobe Corporation under the trademark “Adobe Photoshop”) can watermark digital images using perceptually adaptive techniques.
The technology detailed herein concerns watermarking an image using the highest practical watermark intensity that can be used without creating visual artifacts. An image is divided into areas. Each area only contains portions of the image where it is appropriate to use a watermark signal having the same energy level.
Some images consist of a series of lines. Such images are often used as the background image on security documents or currency. If an image consists of a series of lines, the technology described herein can be applied by dividing the image into areas where the lines that form the image are within a certain range of widths. An appropriate watermark intensity is then determined for each such area and a watermark of this intensity is applied to the image. Watermarks can be applied to such images using the line width modulation techniques shown in co-pending U.S. applications Ser. No. 09/074,034, filed May 6, 1998 which corresponds PCT/US99/08252, and Ser. No. 09/127,503, filed Jul. 31, 1998 which corresponds to PCT/US99/14532.
If an image is a grayscale image, areas having the same tonal value or tonal density are determined directly, or the image is transformed into a line art image and areas with the same size binary valued areas (lines, dots, squares, etc.) are selected by filtering the image.
After the image has been divided areas which have a line width or tonal density value that is within a certain range of values, an appropriate watermark energy level is assigned to each area. The image is watermarked by changing the value of each bit or pixel in the image by an amount determined by both the value in a payload tile and the intensity value associated with the area in which the bit or pixel lies.
The described technology is applicable to watermarking techniques which change the value of a binary parameter in areas of an image (for example, by modulating the width of lines in a line image) or to watermarking techniques which change the value of a multi-valued parameter in areas of the image (for example, by changing the luminance value of the pixels in an image).